This invention relates generally to the field of deposition reactors and more specifically to an apparatus and method for top removal of granular material from a fluidized bed deposition reactor.
Fluidized bed reactors utilize a bed usually comprising a finely divided valuable catalyst which makes it important to design the reactors to prevent catalyst losses. As a result the practice was developed of requiring a large disengaging height above the bed surface and of using cyclones to capture the fine dust that may leave the reactor and then to return it to the bed. A concept referred to as the total disengaging height, (TDH) was developed to estimate the height required for all the particles that would settle out by gravity to settled out. Internal cyclones were provided at this height to capture the finer dust and return it to the bed. Whenever it is desirable to remove the catalyst from the bed the preferred procedure was to remove it from the bottom of the reactor by gravity. Other types of reactors called dilute phase or transport reactors entrained all the solids up through the reactor and out the top, but these reactors did not have a recognizable bed. When these gas-solids reactor concepts are applied to the design of deposition reactors, where gases are introduced to make the bed of particles expand, the dilute phase reactor had a major problem of producing primarily a fine dust which was undesirable. As a result, the majority of deposition reactors have been fluidized beds with a large disengaging height and bottom solids outlets. Internal cyclones have seldom been used because of particle deposition on the outside of the cyclones and the problems of reintroducing particles without plugging the cyclone outlets. Since some fine dust is always produced, most deposition reactors have external cyclones or filters to trap the dust and prevent damage to the equipment used to recover the effluent gases. Thus, the historic approach has been to remove the product from the bottom, provide a large disengaging height to minimize product loss, and use external dust removal.
In 1973, Professor D. Geldart proposed the grouping of powders in to four groups, designated as “Geldart Groups”. The groups are defined by their locations on a diagram of solid-fluid density difference and particle size (see FIG. 5). Fluidized beds are designed based upon the particle's defined by Geldart grouping, which are as follows.                Group A particle size is between 20 and 100 um, and the particle density is typically 1400 kg/m3. Beds from Group A particles will expand by a factor of 2 to 3 at incipient fluidization, due to a low bulk density prior to the initiation of a bubbling bed phase. Most powder-catalyzed beds utilize Group A.        Group B particle size lies between 40 and 500 um and the particles have a density between 1400 and 4500 kg/m3. Bubbling typically occurs at incipient fluidization.        Group C contains extremely fine particles (20 to 30 um) providing the most cohesive particles. These particles fluidize under very difficult to achieve conditions, and may require the application of an external force, such as mechanical agitation.        Group D particles are above 600 um in size and typically have high particle densities. Because of the large particle sizes, fluidization of this group requires very high fluid energies typically associated with high levels of abrasion between bed particles. They are usually used in shallow beds or in a spouting mode.        
A primary use for deposition reactors is to produce high purity silicon. Lord in U.S. Pat. No. 6,451,277 describes, and shows in FIG. 1b, a bed heating method which removes beads from near the top of the bed and then heats them and returns them to the bed. The product 3, is removed from the bottom of the reactor. This bed heating method is rejected in the '277 patent in favor of a preferred option where the beads are removed by gravity from the bottom then reheated and returned to the bed in a pulsed mode. Lord U.S. Pat. No. 6,827,786 provides a detailed description of a multistage deposition reactor which takes advantage of increased bed height to produce additional silicon by use of additional gas injection points along the side of the reactor. In this design the seed generation by grinding is spread out along the reactor because of the extra nozzles and some deposition occurs further from the inlet, but most of the grinding and deposition occurs in the bottom where the solid product is removed. The Lord '786 patent discusses, at Col 3, line 25, the “De Beers” paper which showed the need for some residence time and temperature to fully crystallize the product and dehydrogenate the beads. This is accomplished in the pulsed bead heater at high temperature and with short residence time. The Lord patents and the many references cited do not discuss energy recovery from the effluent gas. However, U.S. Pat. Nos. 5,798,137 and 6,451,277 discusses the use of the heat from the outgoing product to heat the incoming gas.
The primary deficiency of the prior technology is utilizing a fluid bed design with a bottom outlet and large disengaging space and accepting the inherent conflicting demands caused by introducing the cold deposition gas, which also provides the bulk of the seed generation by grinding, at the same location as where the hot product is removed. Lord, in various patents, attempts to deal with the heat and seed generation problem by spreading out the gas injection, but sufficient gas to fully fluidize the bed must be injected at the bottom so there is a limit to what can be accomplished in this manner. Inevitably, the bottom temperature must be maintained above 800° C. to provide the needed crystallization and some seeds are lost to the product which is in turn contaminated with broken “seed beads.” The combination of high temperature and high deposition gas concentration leads to rapid reactions, increased wall deposits and increased risk of agglomeration and plugging.
This multistage design approach also leads to tall reactors and there are cost and manufacturability issues in producing the high purity liners for such reactors which restrict the number of stages and hence production capacity of a given diameter reactor. It is also necessary to measure the bed level and take corrective action by removing some of the bed as the bed grows by opening valves and changing purge flows to allow the right amount of beads to leave the bed. Errors or stuck valves can lead to situations where the bed is too high or too low. Both of these conditions are undesirable.